In recent years, hemoglobin and myoglobin have been shown to be the major source of dietary iron in man. Heme released from these hemoproteins by intraluminal proteolysis is absorbed intact by the mucosal cells, wherein a presumed enzymatic mechanism cleaves the ferroprotoporphyrin ring, setting iron free. This "heme-splitting" mechanism has been tentatively equated with intestinal xanthine oxidase. A more likely possibility is that microsomal heme oxygenase (MHO), a cytochrome P-450 containing enzyme which cleaves heme in liver, spleen, macrophages, and other tissues, is responsible for intramucosal heme- splitting activity. Supporting this concept, our preliminary data have documented the presence of MHO in rat intestine, with highest activity in duodenum. Furthermore, in iron deficiency, duodenal enzyme activity was found to rise, together with increased absorption of hemoglobin- iron. In our research plan, we propose to: 1) characterize intestinal heme oxygenase in terms of its activity, subcellular localization, enzyme kinetics, cofactor requirements, and substrate specificity; 2) define its role in cleavage of the absorbed heme moiety under conditions of altered iron requirements. Rat intestinal MHO activity will be correlated with heme cleavage activity both in mucosal homogenates and in isolated intestinal loops, and with hemoglobin-iron absorption; 3) identify the regulatory factors that may modulate the mucosal enzyme activity under various nutritional and metabolic conditions, and 4) investigate the hypothesis that the adaptive regulation of MHO may be a major factor in the physiological control of hemoglobin-iron absorption.